1. Field of the Invention
The present invention pertains to a method of pattern etching a copper layer on the surface of a semiconductor device substrate.
2. Brief Description of the Background Art
In the multi level metallization architecture used in present day semiconductor devices, aluminum is generally used as the material of construction for interconnect lines and contacts. Although aluminum offers a number of advantages in ease of fabrication, as integrated circuit designers focus on transistor gate velocity and interconnect line transmission time, it becomes apparent that copper is the material of choice for the next generation of interconnect lines and contacts.
Copper has not been used in the past principally because of fabrication problems. In particular, copper is difficult to etch and therefore device patterning is particularly challenging. Known art in copper patterned etching has been encumbered by selectivity and etch rate issues. Etch rates obtained by purely physical bombardment have been typically about 300 .ANG.-500 .ANG. per minute or less, as described by Schwartz and Schaible, J. Electrochem. Soc., Vol. 130, No. 8, p. 1777 (1983) and by H. Miyazaki et al., J. Vac. Sci. Technol. B 15(2) p.239 (1997), respectively. To improve etch rate, various chemical reactants have been used during the etch process. These chemical reactants react with the copper to create volatile species which can then be removed by application of vacuum to the process chamber. However, when such chemical reactants are used, corrosion is a major problem during the fabrication, as copper does not form any self passivating layer like aluminum does. In particular, oxidation of copper increases resistivity; further, in the case of copper interconnect lines, the whole wire line may corrode all the way through, resulting in device failure. Another problem with copper is diffusion into adjacent materials. Typically, a barrier layer is used between copper and adjacent materials, since diffusion of copper into such materials may degrade the material performance properties.
Offsetting the diffusion disadvantage, copper offers improved electromigration performance over aluminum. In fact, in tests to determine the lifetime of an interconnect wire, the electromigration lifetime of a copper wire is approximately 10 times longer than that of aluminum. In addition, as semiconductor device speeds and function are increased, interconnect speed has become critical, and the improved interconnect transmission rates which can be achieved with copper have encouraged circuit designers to look to copper as the solution.
There are two principal competing technologies under evaluation by material and process developers working to enable the use of copper. The first technology is known as damascene technology. In this technology, a typical process for producing a multilevel structure having feature sizes in the range of 0.5 micron (.mu.m) or less would include: blanket deposition of a dielectric material; patterning of the dielectric material to form openings; deposition of a diffusion barrier layer and, optionally, a wetting layer to line the openings; deposition of a copper layer onto the substrate in sufficient thickness to fill the openings; and removal of excessive conductive material from the substrate surface using chemical-mechanical polishing (CMP) techniques. The damascene process is described in detail by C. Steinbruchel in "Patterning of copper for multilevel metallization: reactive ion etching and chemical-mechanical polishing", Applied Surface Science 91 (1995) 139-146.
The competing technology is one which involves the patterned etch of a copper layer. In this technology, a typical process would include deposition of a copper layer on a desired substrate (typically a dielectric material having a barrier layer on its surface); application of a patterned hard mask or photoresist over the copper layer; pattern etching of the copper layer using wet or dry etch techniques; and deposition of a dielectric material over the surface of the patterned copper layer, to provide isolation of conductive lines and contacts which comprise various integrated circuits. An advantage of the patterned etch process is that the copper layer can be applied using sputtering techniques well known in the art. The sputtering of copper provides a much higher deposition rate than the evaporation or CVD processes typically used in the damascene process, and provides a much cleaner, higher quality copper film than CVD. Further, it is easier to etch fine patterns into the copper surface and then deposit an insulating layer over these patterns than it is to get the barrier layer materials and the copper to flow into small feature openings in the top of a patterned insulating film.
Each of the above-described competing technologies has particular process problems which must be solved to arrive at a commercially feasible process for device fabrication. In the case of the damascene process, due to difficulties in the filling of device feature sizes of 0.25 .mu.m and smaller (and particularly those having an aspect ratio greater than one) on the surface of the dielectric layer, the method of choice for copper deposition is evaporation (which is particularly slow and expensive) or chemical vapor deposition, CVD (which produces a copper layer containing undesirable contaminants and is also a relatively slow deposition process). Further, the CMP techniques used to remove excess copper from the dielectric surface after deposition, also create problems. Copper is a soft material which tends to smear across the underlying surface during polishing. "Dishing" of the copper surface may occur during polishing. As a result of dishing, there is variation in the critical dimensions of conductive features. Particles from the slurry used during the chemical mechanical polishing process may become embedded in the surface of the copper and other materials surrounding the location of the copper lines and contacts. The chemicals present in the slurry may corrode the copper, leading to increased resistivity and possibly even corrosion through an entire wire line thickness. Despite the number of problems to be solved in the damascene process, this process is presently viewed in the industry as more likely to succeed than a patterned copper etch process for the following reasons.
The patterned etch process particularly exposes the copper to corrosion. Although it is possible to provide a protective layer over the etched copper which will protect the copper form oxidation and other forms of corrosion after pattern formation, it is critical to protect the copper during the etch process itself to prevent the accumulation of involatile corrosive compounds on the surface of the etched copper features. These involatile corrosive compounds cause continuing corrosion of the copper even after the application of a protective layer over the etched features.
Wet etch processes have been attempted; however, there is difficulty in controlling the etch profile of the features; in particular, when the thickness of the film being etched is comparable to the minimum pattern dimension, undercutting due to isotropic etching becomes intolerable. In addition, there is extreme corrosion of the copper during the etch process itself.
Plasma etch techniques provide an alternative. A useful plasma etch process should have the following characteristics: It should be highly selective against etching the mask layer material; it should be highly selective against etching the material under the film being etched; it should provide the desired feature profile (e.g. the sidewalls of the etched feature should have the desired specific angle); and the etch rate should be rapid, to maximize the throughput rate through the equipment. Typically, a chlorine-comprising gas is used in the reactive ion etch processing of the copper. Although the chlorine provides acceptable etch rates, it causes the copper to corrode rapidly. The chlorine reacts very fast, but produces reaction byproducts which are not volatile. These byproducts remain on the copper surface, causing corrosion over the entire etched surface. The byproducts can be made volatile subsequent to the etch step by treatment with chemical species which create a volatile reaction product, but by this time the corrosion is already extensive.
An example of a treatment to remove chlorides and fluorides remaining after the etch of a conductive layer is provided in U.S. Pat. No. 4,668,335 to Mockler et al., issued May 26, 1987. In Mockler et al., the workpiece (wafer) is immersed in a strong acid solution, followed by a weak base solution after the etch of an aluminum-copper alloy, to remove residual chlorides and fluorides remaining on the surface after etching. Another example is provided in U.S. Pat. No. 5,200,031 to Latchford et al., issued Apr. 6, 1993. In Latchford et al, a process is described for removing a photoresist remaining after one or more metal etch steps which also removes or inactivates chlorine-containing residues, to inhibit corrosion of remaining metal for at least 24 hours. Specifically, NH.sub.3 gas is flowed through a microwave plasma generator into a stripping chamber containing the workpiece, followed by O.sub.2 gas (and optionally NH.sub.3 gas), while maintaining a plasma in the plasma generator.
Attempts have been made to reduce the corrosion by introducing additional gases during the etch process (which can react with the corrosion causing etch byproducts as they are formed). In addition, gaseous compounds which can react to form a protective film over the sidewalls of etched features as they are formed have been added during the etching process and after the etch process. However, residual corrosion continues to be a problem and the protective film, while protecting from future contact with corrosive species, may trap corrosive species already present on the feature surface.
An example of the formation of a passivating film on pattern sidewalls is presented by J. Torres in "Advanced copper interconnections for silicon CMOS technologies", Applied Surface Science, 91 (1995) 112-123. Other examples are provided by Igarashi et al. in: "High Reliability Copper Interconnects through Dry Etching Process", Extended Abstracts of the 1994 International Conference on Solid State Devices and Materials, Yokohama, 1994, pp.943-945; in "Thermal Stability of Interconnect of TiN/CuTiN Multilayered Structure", Jpn. J. Appl. Phys. Vol. 33 (1994) Pt. 1, No. 1B; and, in "Dry Etching Technique for Subquarter-Micron Copper Interconnects", J. Electrochem. Soc., Vol. 142, No. 3, March 1995.
If the patterned etch technique is to be used for fabrication of semiconductor devices having copper interconnects, contacts, and conductive features in general, it is necessary to find an etch method which does not create corrosion or a source of future corrosion during the etch process itself.
In addition to controlling corrosion, it is necessary to control the profile of the etched pattern. An example of a technique used for obtaining a high etch rate and highly directional reactive etching of patterned copper films copper is described by Ohno et al in "Reactive Ion Etching of Copper Films in a SiCl.sub.4, N.sub.2, Cl.sub.2, and NH.sub.3 Mixture", J. Electrochem. Soc., Vol. 143, No. 12, December 1996. In particular, the etching rate of copper is increased by increasing the Cl.sub.2 flow rate at temperatures higher than 280.degree. C. However, the addition of Cl.sub.2 is said to cause undesirable side etching of the Cu patterns. NH.sub.3 is added to the gas mixture to form a SiN-like protective film that prevents side etching. The etch gas mixture which originally contained SiCl.sub.4 and N.sub.2 was modified to contain SiCl.sub.4, N.sub.2, Cl.sub.4, and NH.sub.3. Thus, protective films formed during etching are used by some practitioners skilled in the art to reducing corrosion (as described above) and by others for controlling the directional etching of the pattern surface.
In the above examples, it is presumed that reactive ion etching which includes the use of a chemical reactant for copper is the technique by which dry etching must be accomplished, and considerable effort is put into techniques for removing the reactive ion etch byproducts from the surface of the patterned copper after etch. Further, at the concentrations of the chemically reactive ions recommended for the etching process, there are problems in controlling the directional etching of the copper film.